1. Field of the Invention
The present invention relates to a magnetic type angle sensor using magnetism to sense a rotational angle and rotational position of a rotary shaft or other rotary member.
2. Description of the Related Art
As a magnetic type angle sensor using magnetism to sense the rotational angle of a rotary member, there is known a magnetic type encoder or the like for sensing the rotational position and rotational angle of a motor shaft, a rotary shaft driven by a motor, or another rotary member.
FIGS. 1A to 1C are views for illustrating the operating principle of an angle sensor using magnetism to sense a rotational angle. A rotary body (hereinafter referred to as a “sensor ring”) 1 of an angle sensor mounted on a rotary shaft, etc. is comprised of pure iron or another magnetic material and is formed in its circumferential direction with projecting parts 2 and recessed parts 3 alternately at equal intervals. In FIGS. 1A to 1C, this circular sensor ring 1 is illustrated in a linear form. A magnet 4 is arranged facing the recessed parts and projecting parts formed on the outer circumferential surface of the sensor ring 1. Further, a magnetic sensing device is arranged between the magnet 4 and the recessed and projecting outer circumferential surface of the sensor ring 1. The magnetic sensing device is provided with two magnetoresistance elements 5a and 5b changing in resistance along with the flux density passed therethrough. The magnetoresistance elements 5a and 5b are connected serially to form a series circuit, to which a voltage VCC is applied. The voltage between the two magnetoresistance elements 5a and 5b is sensed as the output voltage.
In the state of FIG. 1A, the magnetoresistance element 5a faces the recessed part 3 of the sensor ring 1, while the magnetoresistance element 5b faces a projecting part 2. As a result, since the magnetoresistance element 5b faces the projecting part 2, the magnetic flux emitted from the magnet 4 which is passes through the magnetoresistance element 5b is great, while since the magnetoresistance element 5a faces the recessed part 3, the magnetic flux which is passed through the magnetoresistance element 5a is small. As a result, the voltage between the magnetoresistance element 5a and the magnetoresistance element 5b, that is, the output voltage, becomes higher.
On the other hand, as shown in FIG. 1B, when the sensor ring 1 moves and the center position between the magnetoresistance elements 5a and 5b is aligned with the center of the recessed part 3 of the sensor ring 1, the two magnetoresistance elements 5a and 5b are evenly passed with magnetic flux from the magnet 4, the resistance values thereof become the same, and the output voltage becomes ½ of the supplied voltage VCC.
As shown in FIG. 1C, when the sensor ring 1 further moves and the magnetoresistance element 5a faces a projecting part 2 and the magnetoresistance element 5b faces a recessed part 3, the magnetoresistance element 5a becomes maximum in resistance value, while the magnetoresistance element 5b becomes minimum in resistance value. As a result, the output voltage becomes the minimum value.
Therefore, the output voltage becomes the output of a sinusoidal wave shape along with movement of the sensor ring. By processing this output signal, the rotational position of the sensor ring 1, that is, the rotational angle of the rotary shaft on which the sensor ring 1 is mounted, is sensed.
Further, in order to determine the position serving as the reference for the rotational angle (origin position), a single pulse origin signal (one-rotation signal) is made to be generated just once when the sensor ring 1 rotates one turn.
FIG. 2 is a view for illustrating the above-mentioned angle sensing signal and origin signal (one-rotation signal). The angle sensing signal is comprised of a large number of pulses generated at equal intervals over one rotation of the sensor ring (one rotation of the rotary member having the sensor ring mounted thereon). The number of these pulses is used for sensing of the rotational angle. On the other hand, the origin signal (one-rotation signal) is comprised of a single pulse output just once every rotation. The position of generation of this pulse is used as the criteria for determination of the rotational reference position of the sensor ring (rotary member having the sensor ring mounted thereon) 1.
In the prior art, as shown in FIG. 5, an angle signal generation sensor ring 20a formed in the circumferential direction with projecting parts 2 and recessed parts 3 alternately at equal intervals and a single rotation signal generation sensor ring 20b were fabricated and attached to a rotary shaft or other rotary member for sensing of the rotational angle. Note that the single rotation signal generation sensor ring 20b shown in FIG. 5 is formed on the circumferential surface thereof with a recessed part 3′ at just a single location, so that it can sense the recessed part 3′ by a magnetoresistance element as described above to generate a one-rotation signal.
Further, Japanese Unexamined Patent Publication No. 11-153451 discloses a magnetic type encoder using a sensor ring integrated with a one-rotation signal generator and an angle signal generator. The sensor ring used in this magnetic type encoder is formed in its circumferential direction with gear shaped recessed parts and projecting parts alternately at equal intervals. One of the projecting parts among these is formed to be shorter in the axial direction thereof to have only about half of the height of the remaining projecting parts. The top half (or bottom half) of the ring in the axial direction thereof is formed with recessed parts and projecting parts alternately at equal intervals. A rotational angle sensing signal is generated at these parts. On the other hand, the other half is formed at just one location with a recessed part. A one-rotation signal is generated at this part. Further, two magnetoresistance elements are used to generate a one-rotation signal from the difference between the outputs of the two magnetoresistance elements.
In the conventional method of separately fabricating an angle signal generation sensor ring 20a and single rotation signal generation sensor ring 20b as shown in FIG. 5 and attaching them to a rotary shaft for sensing of the rotational angle to form an angle sensor, the angle signal generation sensor ring 20a is produced by a machining method such as gear cutting, but the single rotation signal generation sensor ring 20b is produced by a machining method separate from gear cutting, that is, these are fabricated by different machining methods. Accordingly, there is the defect of an increase in the production costs. Further, as the rotary shaft of the motor, etc., has to be fit with the two sensor rings 20a and 20b, two attachment steps are involved and therefore the number of production steps is disadvantageously increased.
Further, the sensor ring integrated with the one-rotation signal generator and angle signal generator, as described in Japanese Unexamined Patent Publication No. 11-153451, eliminates the above-mentioned defects, but generation of the one-rotation signal requires two magnetoresistance elements. As a result, the cost is raised by that extent.